Atrial fibrillation (AF) is a type of tachycardia wherein the atria of the heart beats chaotically, thus interfering with efficient cardiac function. Although not fatal, AF can trigger ventricular fibrillation (VF), wherein the ventricles beat chaotically such that there is little or no net flow of blood from the heart to the brain and other organs. VF, if not terminated, is fatal. Hence, it is highly desirable to prevent or terminate AF and VF.
One technique for preventing AF or VF using a pacemaker is to pace chambers of the heart at a rate somewhat faster than the intrinsic heart rate of the patient using a technique referred to as overdrive pacing. Overdrive pacing may be applied to the atria or the ventricles. A particularly effective overdrive pacing technique for the atria, referred to herein as dynamic atrial overdrive (DAO) pacing, is described in U.S. Pat. No. 6,519,493 of Florio et al., entitled “Methods And Apparatus For Overdrive Pacing Heart Tissue Using An Implantable Cardiac Stimulation Device”, issued Feb. 11, 2003. With DAO, the overdrive pacing rate is controlled to remain generally uniform and, in the absence of a tachycardia, is adjusted upwardly or downwardly only occasionally. The aggressiveness of overdrive pacing may be modulated by adjusting the overdrive pacing rate and related control parameters. See: U.S. patent application Ser. Nos. 10/093,225 and 10/092,695, both of Florio et al., entitled “Method And Apparatus For Using A Rest Mode Indicator To Automatically Adjust Control Parameters Of An Implantable Cardiac Stimulation Device”, and both filed Mar. 6, 2002; U.S. patent application Ser. No. 10/043,781, also of Florio et al., entitled “Method And Apparatus For Dynamically Adjusting A Non-Linear Overdrive Pacing Response Function”, filed Jan. 9, 2002; and U.S. patent application Ser. No. 10/043,472, of Falkenberg et al., entitled “Method And Apparatus For Dynamically Adjusting Overdrive Pacing Parameters”, filed Jan. 9, 2002. Capture of overdrive pulses may be verified as set forth in U.S. patent application Ser. No. 10/138,438, of Bradley et al., entitled “Method And Apparatus For Providing Atrial AutoCapture In A Dynamic Atrial Overdrive Pacing System For Use In An Implantable Cardiac Stimulation Device”, filed May 2, 2002. Each of the aforementioned patent applications is incorporated herein in their entirety.
It is believed that DAO and DVO are effective for at least some patients for preventing AF and VF for the following reasons. A normal, healthy heart beats only in response to electrical pulses generated from a portion of the heart referred to as the sinus node. The sinus node pulses are conducted to the various atria and ventricles of the heart via certain, normal conduction pathways. In some patients, however, additional portions of the heart also generate electrical pulses referred to as “ectopic” pulses. Each pulse, whether a sinus node pulse or an ectopic pulse has a refractory period subsequent thereto during which time the heart tissue is not responsive to any electrical pulses. A combination of sinus pulses and ectopic pulses can result in a dispersion of the refractory periods, which, in turn, can trigger a tachycardia such as fibrillation. By overdrive pacing the heart at a generally uniform rate slightly above the intrinsic rate, the likelihood of the occurrence of ectopic pulses is reduced, the refractory periods within the heart tissue are rendered more uniform and periodic and the heart is thereby resynchronized. Hence, the dispersion of refractory periods is reduced and the risk of AF or VF is reduced.
Thus, overdrive pacing, particularly DAO and DVO, provides a useful technique for helping to prevent the onset of a tachycardia such as fibrillation. As noted, the aggressiveness of overdrive pacing may be adjusted by modulating the overdrive pacing rate and related control parameters. Circumstances may arise, however, where it is desirable to modulate the aggressiveness of overdrive pacing by instead adjusting the magnitude of the overdrive pulses or by changing the electrodes with which the pacing pulses are delivered to the heart. For example, there may be a need to ensure that the overdrive pulses are captured in a larger portion of heart tissue so as to achieve enhanced resynchronization. Aspects of the invention are directed to this end. In particular, it would be desirable to provide at least two “tiers” of overdrive pacing therapy, employing different pulse magnitudes and different electrode combinations, to thereby allow the scope of pulse capture to be controlled.
Whereas overdrive pacing is primarily directed to preventing tachycardias such as AF from arising, cardioversion is employed to terminate AF once it has occurred. Patients prone to AF may have an ICD implanted therein that is capable of detecting AF and automatically administering one or more cardioversion shocks in an effort to revert the atria to a normal sinus rhythm. Typically, the ICD administers about two joules of energy directly to the atria in each cardioversion shock. Cardioversion techniques are described in U.S. Pat. No. 6,445,949 to Kroll, entitled “Implantable Cardioversion Device with a Self-Adjusting Threshold for Therapy Selection”, which is incorporated by reference herein.
Although cardioversion is effective in terminating AF, in many cases fibrillation soon returns, requiring another round of shocks. Repeated shocks are quite painful and can deplete battery resources of the implanted device. One reason cardioversion shocks are painful is that the patient is typically conscious and alert at the time the shock is administered. This is in contrast with much stronger defibrillation shocks provided for terminating VF, which are typically not administered until the patient has lost consciousness. Because AF is not usually immediately life threatening, painful shocks for its treatment may be perceived by patients as worse than the disease itself and therefore not tolerated. Indeed, anxiety arising in a patient from the fear of receiving multiple, painful cardioversion shocks may be sufficient to raise the heart rate sufficiently to trigger such shocks.
As some patients have hundreds of AF episodes annually, it would be highly desirable to provide techniques for preventing the re-occurrence of AF following a cardioversion shock so as to reduce the need for multiple, repeated cardioversion shocks. It is to this end that other aspects of the invention are directed. In particular, it would be desirable to provide techniques for coordinating the delivery of cardioversion therapy and overdrive pacing therapy so as to prevent the re-occurrence of AF following a cardioversion shock.